I say it is 1/4, 1/8 was only made on 35x magnification, and later on 8-32x40
I have a couple of 6-24 4200s. One has a dot and the other has a duplex reticule. Both of the scopes are 1/8 minute adjustments. If you have short turrets you have 1/8 minute adjustments.
Minshall,
Under the turret are a series of lines (and sometimes numbers) that are generally used to allow you to re zero your rifle after having made a scope adjustment.
The lines refer to a few things but the idea is to zero your rifle (at 100 yards for instance) and then carefully remove the cap and re set it with the zero on the cap aligned with the small vertical line (on both up down and left right adjustments). Then after your scope is set to zero you will need to make note of what horizontal line the bottom of the turret cap aligns with. Your notes can be simple such as
Base zero Third line from bottom Turret cap zero
200 yard zero Third line from bottom Turret cap 3.25
300 yard zero Fifth line from bottom Turret cap 3.5
Once you have the zero ranges the idea is to always return "home" to your base zero after you took that 300 yard shot at a coyote (or whatever).
This should get you started though later on you will probably want to add more detailed drop charts.
Good luck with your new toy.